Multi-center, phase 3 study conducted in three parts. Part 1 is a randomized, placebo-controlled, double-blind study to determine the efficacy and safety of C1-Inhibitor (Human) as a therapeutic agent for acute attacks of HAE. Completion of Part 1 confers eligibility for participation in either Part 2 (open-label, treatment on demand) or Part 3 (prophylaxis prior to surgical intervention). Background: Hereditary angioedema is an autosomal-dominant disorder associated with serum deficiency of functional C1 inhibitor, the only inhibitor for C1 esterase of the complement cascade and the coagulation factors XIa and XIIa. It is also a primary inhibitor for kallikrein. Clinical symptoms of HAE include episodic swelling of the extremities, face, and bowel wall. Involvement of the upper airway, with resultant asphyxia, can be a cause of death among HAE patients. Androgens and antifibrinolytics are used to inhibit the occurrence of HAE attacks. Though usually effective in prophylaxis, these agents are unable to prevent the progression of an acute HAE attack. Only replacement therapy can stop the progression of an acute HAE attack. In early investigations in the late 1960s and early 1970s, fresh frozen plasma was succesfully used to treat acute attacks of HAE. Since laryngeal attacks are life-threatening and can be triggerd by oral trauma, fresh frozen plasma (FFP) has also been used prophylactically prior to dental surgery. In Europe, where C1-inhibitor concentrates have been available over the past decade, they are preferred over FFP in the treatment of acute HAE attacks. Plasma contains activated complement components and kinin factors which can worsen symptoms. FFP also present a greater risk of infection with blood-borne viruses since it is not subject to dedicated viral inactivation procedures. C1-inhibitor (Human) vapor heated has been commercially available in Europe for approximately 10+ years. Donor screening, extensive partitioning during manufacture and a proprietary vapor-heating treatment are used to render a product with little risk of viral transmission. At present, there is no licensed C1-inhibitor (Human) for acute attaks of HAE in the U.S.